Method of determining amount of fluid in underground storage



INVENTOR.

J.H. ALLEN A 7'7'ORNEKS CROSS REFEENC J. H. ALLEN Filed May 29, 1958METHOD OF DETERMINING AMOUNT OF' FLUID IN UNDERGROUND STORAGE Aug. 21,1962 United States Patent O 3,049,920 METHOD OF DETERMINING AMOUNT OFFLUID IN UNDERGROUND STORAGE James H. Allen, New York, N.Y., assignor toPhillips Petroleum Company, a corporation of Delaware Filed May 29,1958, Ser. No. 738,663 8 Claims. (Cl. 73-291) This invention relates tothe underground storage of fluids such as gaseous ethylene, liquefiedpetroleum gases such las propane and butane, and the like. 'In oneaspect, it relates to a method of obtaining measurements and data usefulin determining the amount of such fluids stored as products in anunderground storage cavern, such as those caverns formed below theground in salt formations and the like.

Constantly expanding production of fluids for the industries of thiscountry and elsewhere has created a denite problem in providing suitablestorage facilities for these fluids. In lpetroleum industries, inparticular, the problem of storage of fluids such as gaseous ethylene,liquefied petroleum gases such as propane and butane, ammonia, and thelike, is presently an urgent one due to the high cost of storage insurface equipment, such as steel tanks, and due to the massiveconstruction required to withstand the vapor pressure of such fluids.Also adding to the problem of adequate storage facilities is the factthat many industries, especially the liquefied petroleum gases industry,experience seasonal peak loads in `the requirements for their .productsand corresponding seasonal slack periods. These fluctuations inrequirements require large storage facilities and the advantages ofstoring fluids in underground caverns have lately become very attractiveand important.

Underground storage caverns are generally formed in impermeable earthformations, either by conventional mining methods, or, in some cases, bydissolving out a soluble material with solvents to create a storagespace in soluble formations, 4for example, in salt domes. The resultingcaverns are less expensive to provide than would be an equal volume oforthodox sur-face space and have particularly proven their -value in thestorage of liquefied petroleum gases.

The instant invention is particularly concerned with those undergroundstorage caverns formed in underground salt formations. This type ofcavern is generally formed by drilling an access bore from the surfaceof the ground down into a salt formation, such as a salt dome, and thenwashing out the salt by circulating fresh water down one conduit in theaccess bore to dissolve the salt, while continuously removing theresulting brine through another conduit in the access bore. Afterformation of the cavern, product to be stored therein, such as gaseousethylene or liquefied petroleum gases, is pumped into the cavern undersufficient pressure to displace the brine in the cavern. This isgenerally done by pumping the product into the annulus between thecasing and a central pipe, commonly called an eductor pipe. The brine isgenerally forced to the surface through the eductor pipe. The product,being lighter than and immiscible with the brine in .the cavern,occupies a space in the cavern above the pool of brine, an interfacebeing formed between the two fluids. When it is desired to remove theproduct Ifrom storage, brine is generally forced into the cavern via theeductor pipe, thereby displacing the stored product through the annulusbetween the eductor pipe and casing.

One of the problems involved in this type of storage is that ofdetermining the amount of product stored in the cavern. These cavernsare generally located hundreds or even thousands .of feet below the-surface of the ground in regions which are inaccessible to an observer.The

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access bore is, of course, relatively small and this limits the mannerin which the amount of product stored in the cavern can be determined.Then, again, the cavern often has generally a somewhat irregular shapewhich itself presents problems. During storage, there will generallyalways be a pool of brine in the bottom of the cavern and this lfurthercomplicates the determination of the amount of product in the cavern.Also, variations in such factors as temperature, pressure, and densityof the stored product must be taken into account if a true and accuratedetermination is to be made of the amount of stored product; the deeperthe cavern, the more effect will these factors have on the physicalproperties of the stored product.

Determining the amount of stored product by metering into and out of thecavern the product and displacing fluid has heretofore proven to begenerally an unsatisfactory method of determining the amount of storedproduct. Metering methods are often unreliable because of errors inmetering, especially where the product is stored over a relatively longperiod during which losses to the formation and errors in meteringcalibration can occur. These metering methods have especially been foundwanting in those cases where it is desirable to periodically determinethe exact amount of stored product because these errors are cumulative;after a period of months in which the product has been withdrawn andadded, one no longer has confidence in his knowledge of the amount ofproduct stored in the cavern.

Accordingly, it is an object of this invention to provide an improvedmethod of determining the amount of product stored in an undergroundstorage cavern. Another object is to provide an improved mehod forobtaining measurements and data useful in determining the amount of afluid, such as gaseous ethylene, liquefied petroleum gases such aspropane and butane, and the like, in an underground storage cavern,particularly in a cavern formed in a soluble salt formation. Anotherobject is to minimize errors in measuring -factors necessary indetermining the true amount of product stored in an underground storagecavern. Other objects and advantages of this invention will becomeapparent from the following discussion, appended claims, andaccompanying drawing i-n which a schematic elevational View in partialsection is shown of an underground storage cavern provided 'withappurtenances necessary in determining the amount of product stored inthe cavern in accordance with this invention.

Referring now to the drawing, an underground storage cavern generallydesignated 10 is illustrated. This cavern 10 can be formed by methodswell known in the art. For example, a vertical access bore 11 is-drilled from the ground through various overlying formations 12, suchas surface soil, shale, limestone, sandstone, and the like, into asoluble formation, such as a salt formation 4or salt dome 13, the latter.generally having thereabove a layer of cap rock such as anhydrite orgypsum, and preferably drilling the access bore to the ultimate depth ofthe subsequently formed cavern. After drilling the access bore 11 intothe salt formation 13, a casing 14 is set in the borehole and cementedat 16 to the surrounding Ifo-rmation so as to form a Ifluid-tight sealagainst the leakage of fluids past the casing and to securely anchorsaid casing. A pipe 17, such as an eductor or wash pipe, is insertedthrough casing 14 in Vspaced relation thereto, thereby forming anannulus 18, the lower end of pipe 17 depending beneath the lower end ofcasing 14 and extend ing further into the Vaccess bore. Alternatively,another pipe (not shown) can be inserted in .the borehole concentricallysurrounding pipe 17, this other protective pipe depending below thebottom of casing 14 and above the Elower end of pipe *17, this otherpipe serving to protect the inner pipe 17 during the washing operation.The upper end of casing 14 can be provided with surface pipe 19 having aflow control valve 21 and pressure gauge 22 afixed thereto. The upperend of the inner pipe 17 can also be provided with a branched conduit 23having n valve 24 and pressure gauge 26 affixed thereto.

In forming the cavern, a solvent such as fresh water is pumped downinner pipe 17 so as to contact the soluble salt formation 13. Theresulting pool of brine 27 is thereby formed and it is forced up throughthe annulus 18 to the surface of the ground where it is removed viaconduit 19. During the formation of the cavern, the inner pipe 17 can bevertically moved up and down in order to facilitate the washingoperation. Occasionally, the circulation of the fluids in the boreholeand the cavern can be reversed, that is, fresh water can be pumped downthrough annulus 18 and brine removed from the cavern via inner pipe 17.It is also advisable in many cases to protect the roof of theprogressively formed cavern and the foot of the casing pipe during thewashing operation by injecting a protective blanket of hydrocarbon, suchas diesel fuel, L.P.G., or even the subsequently stored product, intothe cavern in such a manner that it floats on top of the wash solutionor pool of brine in the cavern, the product being lighter than andimmiscible with the brine. The cavern resulting from the washingoperation will very often have an irregular shape such as that shown inthe drawing, this shape being due to the manner of circulation, presenceof shale stringers, or insoluble material, such as gypsum, embedded inthe salt formation 13.

After the underground storage cavern 10 has been formed, it is generalpractice to survey its shape and size. This can be accomplished byseveral methods, such as that disclosed in U.S. Patent 2,792,708, issuedMay 21, 1957, to R. W. Johnston, Ir., et al., or any other known method.A particularly useful method which may be used to measure the shape andsize of the cavern is that provided by the services of DowellIncorporated, wherein a sonic caliper (an electric-line tool) isemployed. This caliper works on pulse-echo system, timing the rate ofsound through a fluid medium. It is an adaptation of the devices used insonic navigation and ranging. In operation, sound waves are emitted froma sonic device or impulse emitter and travel out through the uid mediumin the cavern and are reflected back as echoes from the wall definingthe cavern to a sound sensitive receiving device. The time intervalbetween transmission and reception of the sound pulse is a `directmeasure `of the distance to the wall of the cavern. The transmitting andreceiving devices are contained in a slender tool which is lowered bymeans of an electric line. By incremental lowering of the tool andmaking these measurements the diameters of the horizontal cross sectionsof the cavern are found and correlated with depth to give a profile ofthe cavern and an indication of its size. When making this type ofsurvey, the inner pipe 17 is of course removed from the access bore.

From the above survey, it is possible to determine the incrementalvolume of the cavern from top to bottom; this will be done in incrementsof 5 or 10 feet. After determining the shape and size of the cavern 10,product 28 can then be stored in the cavern. Generally, this productwill be injected down the annular space 18 and brine will be therebydisplaced from the cavern via the inner pipe 17. From a knowledge of theshape and size of the cavern 10, the amount of product stored in thecavern can be readily determined by making the measurements according tothe practice of this invention.

According to this invention, the interface 31 formed by the pool ofbrine 27 and the stored product 28 thereabove is determined in thefollowing manner. An interface detecting device 32 is lowered in innerpipe 17 through a valve 33 afxed to the top of the inner pipe 17 bymeans of a wire line or insulated electrical conducting cable 34. Thecable or line 34 extends through a stuffing box 35 and passes over adepth measuring sheave 36 and is then wound on a storage or hoist drum37 which may be actuated by any suitable means. The interface detectingdevice employed can be any of the devices known to be useful formeasuring the interface between two immiscible iiuids, such as thatdisclosed in U.S. Patent 2,648,778, issued August 11, 1953, to D.Silverman et al., the device disclosed therein being capable ofdetecting the interface between two dissimilar fluids by reason ofvariations in gamma-ray absorption. A particularly useful interfacedetect-ing device is a gamma-gamma log device provided by the serviceoffered by the Lane-Wells Company. Once the winterface 31 is detectedand its depth recorded, the amount or volume of space in the cavern 10occupied by the stored product can be readily determined by correlatingthe interface depth with the previously run cavern survey.

After locating interface 31, a temperature survey is then made of thestored product. According to one method, a temperature recording deviceis lowered in inner pipe 17 in the same manner as that hereinbeforedescribed in regard to the interface detecting device 32. A particularlyuseful temperature recording device is that developed by theSchlumberger Well Survey Corporation, described in Petroleum ProductionEngineering by Lester Charles Uren, 3rd ed., published by McGraw-HillBook company, Inc., New York (1946), pages 656-658. This device is asurface-recording thermometer of the electrical resistance type which islowered on an insulated conductor cable. The temperature recordingdevice is incrementally lowered within the inner pipe 17 and thetemperature recorded from the top of the inner pipe 17 to the interfacedepth.

Following the temperature survey, a pressure survey is made according tothis invention in the following manner. A pressure recording device islowered within the inner pipe 17 in much the same manner as thatdescribed hereinbefore in regard to the lowering of the interfacedetector and temperature recording device. The pressure recording deviceemployed can be any one of those known to be useful in the art forrecording a temperature in a borehole. A particularly useful pressurerecording device is that known as an Amerada Continuous RecordingPressure Gauge described in Petroleum Production by Wilbur F. Cloud,published by the University of Oklahoma Press (1939), at page 207. Thispressure gauge operates on the Bourdon tube principle in conjunctionwith a clock mechanism. With the inner tube 17 filled with brine, thepressure recording device is lowered to the interface depth 31 and thepressure at that point is recorded. The pressure of the gas at gauge 22is also recorded. The pressures are taken at these two points 22 and 31and the pressure gradient therebetween can be readily calculated So asto obtain pressures at predetermined depths by gas law calculations ofthe type explained in the Journal of Petroleum Technology, volume 207,pages 281-287, December 1956. Alternatively, the inner pipe 17 can befilled with product down to the interface level and the pressuredetecting device lowered within the inner tube 17 and pressuresincrementally determined from the top of pipe 17 to the interface depth.

From a knowledge of the shape and size of the cavern, and the interface,temperature, and pressure surveys obtained in the above-describedmanner, the amount of product stored in the underground storage caverncan be readily and accurately determined from the measurements taken.

The advantages of this invention are further illustrated in thefollowing example.

An access bore was drilled from the ground surface into an undergroundsalt dome to a depth of 2840 feet, which depth was about 1400 feet belowthe top of the salt dome. Casing was then run into the access bore to adepth of 2820 feet `and cemented in place. The access bore was thendrilled at a reduced diameter to about the projected total depth of theproposed cavern, for example, 3115 feet. Solution of the salt formationwas then accomplished in a conventional manner using either one or twostrings of tubing to control the Washing action. The tubing was thenwithdrawn from the access bore and cavern. The shape and size of theresulting cavern was then determined by lowering a sonic caliper in thecavern via the casing. The sound generator or sonic device of thecaliper emitted a sound wave which was beamed in a narrow arc toward thecavern wall. The reflected sound wave was picked up by the receiver ofthe caliper and a direct measurement is made of the distance from thecaliper to the wall of the cavern. By rotating the sonic caliper withrespect to the cardinal points of the compass, the distance to thecavern wall in all direction was determined. By repeating thisdetermination at different depths, a complete determination was made ofthe cavern shape and the incremental volume of the cavern from top tobottom was readily calculated.

In the upper portion of the cavern, the incremental volumes of thecavern from 2829 feet to 2918 feet were as follows:

Incremental volume between:

2829' to 2839=l537 cu. ft. 2839' to 2849=1537 cu. ft. 2849 to 2859*:1537cu. ft. 2859 to 2869'=6154 cu. ft. 2869 to 2879=26,007 cu. ft. 2879 to2889=l67,534 cu. ft. 2889' to 2899=163,394 cu. ft. 2899 to 2909=160,595cu. ft. 2909 to 2918=104,820 cu. ft.

The incremental volumes of the lower portion of the cavern below 2918feet were also determined but are not reported above for purposes ofbrevity.

After determining the incremental volumes of the entire cavern, theeductor pipe 17 was then reinserted and installed in the cavern and thesame was filled or partially filled with product, namely gaseousethylene. This latter was introduced into the annulus 18 via surfacepipe 19 and as it entered the cavern it displaced the brine 27 to thesurface via eductor pipe 17 and surface pipe 23, -an interface 31 wasformed between the stored product 28 and brine 27. Various withdrawalsand additions of stored product were made during the course of normaloperations until it was necessary or desirable to make an inventorydetermination of the exact amount of product stored in the cavern.

With the eductor pipe 17 remaining in the cavern, as shown in thedrawing, the depth or location of interface 31 was determined bylowering an interface detector device 32, such as the gamma-gamma logdevice previously described. The depth of the interface was found to be2918 feet.

After detecting and locating the interface, the temperature at each ofthe incremental volumes above the interface was determined by running atemperature survey in the manner described herein-before. For example,the average temperature between 2909 feet 4and 2918 feet was found to be106 F. The pressure survey was run and the average pressure of theproduct between 2909 feet and 2918 feet was found to be 1451.75 p.s.i.The density of ethylene at 1451.75 p.s.i. Iand 106 F. could have beenfound by deriving the same from the gas law PV=ZRT. In practice, thedensities of ethylene at various pressures and temperatures were foundfrom graphs prepared beforehand. From these graphs it was found that thedensity of ethylene at 1451.75 p.s.i. and 106 F. is 16.357 pounds percu. ft.

'Ihe exact amount or quantity of ethylene in the cavern between 2909feet and 2918 feet was readily determined by multiplying theaforementioned density by the incremental volume at this depth, e.g.(16.357 pounds per cu. ft.) X (104,820 cu. ft.)=1,7l4,541 pounds. Theaforementioned procedure was repeated to determine the amount of productin each incremental volume above 2909 6 feet and the summation of theamounts in the incremental volumes to the surface of the ground gave thetotal amount of stored product. This did, of course, include the amountof product stored in annulus 18. l

During subsequent operations involving further withdrawals and additionsof stored product, the amount of stored product can be periodicallydetermined according to the aforementioned procedure of this invention.

Various modications and alterations of this invention will becomeapparent to those skilled in the art without departing from the scopeand spirit of the invention, and it should be understood that the latteris not necessarily limited to the aforementioned discussion andaccompanying drawing.

I claim:

1. In Ia method of determining the amount of fluid stored above a poolof liquid in an underground storage cavern, said stored liuid beinglighter than and immiscible with said liquid, said cavern comprising achamber having known incremental volumes of relatively large diameterand 'an access bore of relatively small diameter extending up from saidchamber to the ground, the physical steps comprising suspending aninterface detecting device within said cavern and locating the interfacebetween said stored fiuid and said liquid, withdrawing said interfacedetecting device from said cavern after locating said interface,suspending a temperature recording device in said cavern and recordingthe temperature of said fluid in each of said incremental volumes abovesaid interface, withdrawing said temperature recording device from saidcavern, suspending a pressure recording device within said cavern anddetermining the pressure of said fluid in each of said increment-alvolumes, and withdrawing said pressure recording device from saidcavern, whereby the amount of said fluid stored in said cavern can becomputed by correlating all of the measurements taken.

2. In a method according to claim 2 wherein said incremental volumes aredetermined while said storage cavern is initially completely filled withsaid liquid by the steps comprising incrementally suspending an impulseemitter and a sound sensitive receiver in said cavern, emitting sonicimpulses from said emitter, receiving echoes of said impulses reflectedfrom the wall defining said cavity, and measuring the time elapsingbetween respective sonic impulses and their echoes as a measurementindicative of the distance traveled by said impulses, whereby theincremental volumes of said cavity can be determined.

3. In a method of determining the amount of fluid stored above a pool ofliquid in a sealed underground storage cavern formed in a soluble,impermeable earth formation, said stored fiuid being lighter than andimmiscible with said liquid, said cavern comprising an irregularlyshaped chamber having varying horizontal cross sections of relativelylarge diameter and an access bore of relatively small diameter extendingup from said chamber to the ground, the physical steps comprisingincrementally and centrally suspending an impulse emitter and a soundsensitive receiver in said cavern while said storage cavern is initiallycompletely filled with said liquid, radially emitting sonic impulsesfrom said emitter through said liquid, receiving echoes of said impulsesreflected from the wall of said formation defining said cavity,measuring the time elapsing between respective sonic impulses and theirechoes as a measurement indicative of the diameter of each incrementalvolume of said cavity, withdrawing said emitter and receiver from saidcavern, filling said cavern with said uid so as to form an interfacebetween said fluid and said liquid, lowering a gamma-ray emitter andgamma-ray sensitive receiver in said cavern, radially emittinggamma-rays from said emitter and recording the differential absorptionof the gamma-rays in said fiuid and liquid so as to locate saidinterface, withdrawing said gamma-ray emitter and receiver from saidcavern, suspending a temperature recording device in said cavern andrecording the temperature of said uid in each of said incrementalvolumes above said interface, withdrawing said temperature recordingdevice from said cavern, suspending a pressure recording device withinsaid cavern and determining the pressure of said iiuid in each of saidincremental volumes, and wi-thdrawing said pressure recording devicefrom said cavern, whereby the amount of said fluid stored in said caverncan be computed by correlating all of the measurements taken.

4. In a method of determining the amount of fluid stored above a pool ofliquid in a sealed underground storage cavern formed in a soluble,impermeable earth formation, said stored fluid being lighter than andimmiscible with said liquid, said cavern comprising an irregularlyshaped chamber having varying horizontal cross sections of relativelylarge diameter and an access bore of relatively small diameter extendingup from said chamber to the ground, the physical steps comprisingincrementally and centrally suspending an impulse emitter and a soundsensitive receiver in said cavern while said storage cavern is initiallycompletely filled with said liquid, radially emitting sonic impulsesfrom said emitter through said liquid, receiving echoes of said impulsesreflected from the wall of said formation defining said cavity,measuring the time elapsing between respective sonic impulses and theirechoes as a measurement indicative of the diameter of each incrementalvolume of said cavity, withdrawing said emitter and receiver from saidcavern, providing said cavern with a conduit extending from the groundto a point adjacent the bottom of said cavern, the upper portion of saidconduit and said access bore defining an annulus, introducing said fluidinto said cavern via said annulus and displacing a portion of saidliquid from said cavern to the ground via said conduit thereby formingan interface in said cavern between said fluid and that portion of saidliquid remaining in said cavern, lowering a gammaray emi-tter andgamma-ray sensitive receiver in said cavern, radially emittinggamma-rays from said emitter and recording the differential absorptionof the gammarays in said fluid and liquid so as to locate saidinterface, withdrawing said gamma-ray emitter and receiver from saidcavern, lowering a temperature recording device in said conduit andincrementally recording the temperature of said fluid therein above saidinterface, withdrawing said temperature recording device from saidcavern, lowering a pressure recording device in said conduit andincrementally determining the pressure of said fluid therein above saidinterface, and withdrawing said pressure recording device from saidcavern, whereby the amount of said fluid stored in said cavern can becomputed by correlating all of the measurements taken.

5. In a method according to claim 3 wherein said fluid is gaseousethylene, said liquid is brine, and said formation is a soluble saltformation.

6. In a method according to claim 3 wherein said fluid is butane, saidliquid is brine, and said formation is a soluble salt formation.

7. In a method according to claim 3 wherein said fluid is propane, saidliquid is brine, and said formation is a soluble salt formation.

8, In a method according to claim 3 wherein said fluid is liquefiedpetroleum gas, said liquid is brine, and said formation is a solublesalt formation.

References Cited in the le of this patent UNITED STATES PATENTS2,792,708 Johnston et al May 21, 1957 2,817,235 Hunter et al Dec. 24,1957 UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent No.3,049,920 August 21E 1962 James H. Allen It is hereby certified thaterror appears n the above numbered pat ent requiring correction and thatthe said Letters Patent should read as corrected below Column line 37,for the claim reference numeral "2" read l Signed and sealed this 1stday of January 1963.

(SEAL) Attest:

DAVID L. LADD ERNEST W. SWIDER Commissioner of Patem Attestng Officer

